The present invention relates to a device and method for compensating for chromatic dispersion of an optical signal through a waveguide, such as an optical fiber, that utilizes a transmissive, long-period grating.
Chromatic dispersion, or the propagation of different frequency components of an optical signal at different velocities, is an increasingly important problem that must be addressed in high-bit-rate fiber-optic communication systems. Unchecked, dispersion leads to broadening and smearing of the features of rapidly varying optical signals and, as a result, a degradation of the bit-error rate at the receiving end of a link occurs.
There are currently two main approaches used for chromatic dispersion compensation and control in fiber-optic systems, including dispersion compensating fiber (DCF) and chirped fiber Bragg gratings (CFBG). A dispersion compensation module is a simple transmissive device, essentially a spool of fiber, that offers a smooth dispersion spectrum. Unfortunately, DCFs have several shortcomings. A small mode size and a high core-clad index difference are often required. In addition, a large length of such fiber is required. All of these factors result in losses and large nonlinear optical effects over the length of fiber required to provide useful dispersion. It is also difficult to tailor the spectral dependence of the dispersion using DCFs. A second approach for compensating for chromatic dispersion is a chirped fiber Bragg grating (CFBG). It is known that dispersion can be caused and also tailored by reflecting an optical signal off a CFBG. In contrast to DCFs, CFBGs are compact, have very low nonlinear optical effects, and provide a very straightforward means to tailor the dispersion spectrum.
Unfortunately, CFBGs also suffer from several shortcomings. CFBGs are inherently reflective devices and, hence, typically require an optical circulator or lossy coupler. Furthermore, because the differential path lengths giving rise to dispersion occur through reflection over millimeters or centimeters of fiber and because of the inherent difficulty in fabricating fibers with an extremely high degree of uniformity over these length scales, CFBGs tend to suffer from xe2x80x9cdispersion ripple,xe2x80x9d a rapid and large variation of the dispersion over a spectral width as narrow as a typical wavelength division multiplexing (WDM) channel spacing.
Clearly, there is a need for a device and corresponding method for compensating for chromatic dispersion that is compact, that has a tailorable dispersion spectrum, a low nonlinear-optical effect, a low loss, and a low dispersion ripple.
The present invention provides a device for compensating for chromatic dispersion that eliminates or at least ameliorates the shortcomings of the prior art.
The device compensates for chromatic dispersion in an optical signal conducted through an optical waveguide of a type having (i) a fundamental-mode waveguide and (ii) a higher-order mode waveguide surrounding the fundamental-mode waveguide, wherein different effective indexes of refraction are associated with the fundamental-mode and higher-order mode waveguides. The device comprises a transmissive optical grating disposed along a length of the optical waveguide and has a receiving end and a transmitting end. The grating is chirped between the ends along a light conducting axis of the optical waveguide such that different wavelengths of the optical signal are coupled between the fundamental-mode and higher-order mode waveguides at different points along the length of the grating such that chromatic dispersion is corrected at the transmitting end.
The present invention also provides a device for compensating for chromatic dispersion in an optical signal having a bandwidth xcex94xcex. The device comprises:
an optical waveguide comprising (i) a fundamental-mode waveguide and (ii) a higher-order mode waveguide, wherein each of the fundamental-mode and higher-order mode waveguides has an effective index of refraction, the optical waveguide further including:
a chirped grating having a length LG that couples light from the fundamental-mode waveguide to the higher-order mode waveguide, wherein the chirp of the grating is calculated by the formula:       c    ⁢          xe2x80x83        ⁢    h    ⁢          xe2x80x83        ⁢    i    ⁢          xe2x80x83        ⁢    r    ⁢          xe2x80x83        ⁢    p    ≅            -                        33          ⁢          Δ          ⁢                      xe2x80x83                    ⁢                      n            eff                                    D          ⁢                      xe2x80x83                    ⁢                      L            F                                ⁢          (              nm        cm            )      
wherein xcex94neff is the difference between the effective indexes of refraction of the fundamental-mode and higher-order mode waveguides and DLF is the dispersion (in ps/nm) to be compensated from a fiber with a dispersion coefficient D and a length LF, and
wherein the length of the grating LG is calculated by the formula:       L    G    ≅      Δλ    chirp  
wherein xcex94xcex is the bandwidth over which compensation is desired
The present invention also provides a method for compensating for chromatic dispersion in an optical signal. The method comprises conducting the optical signal through a device comprising an optical waveguide of a type that has (i) a fundamental-mode waveguide and (ii) a higher-order mode waveguide surrounding the fundamental-mode waveguide, wherein different effective indexes of refraction are associated with the fundamental-mode and higher-order mode waveguides, the device further comprising a transmissive optical grating disposed along a length of the optical waveguide and having a receiving end and a transmitting end, the grating being chirped between the ends along a light conducting axis of the optical waveguide such that different wavelengths of the signal are coupled between the modes of the optical waveguide at different points along the length of the grating such that the dispersion is corrected at the transmitting end.
The invention also provides an apparatus for compensating for chromatic dispersion in an optical signal conducted through an optical waveguide of a type having (i) a fundamental-mode waveguide and (ii) a higher-order mode waveguide surrounding the fundamental-mode waveguide, wherein different effective indexes of refraction are associated with the fundamental-mode and higher-order mode waveguides, the chromatic dispersion having a first-order dispersion and higher-order dispersion. The apparatus comprises a device that compensates the first order dispersion and a device that compensates the higher-order dispersion. The device that compensates the higher-order dispersion comprises a transmissive optical grating disposed along a length of the optical waveguide and having a receiving end and a transmitting end, the grating being chirped between the ends along a light conducting axis of the optical waveguide such that different wavelengths of the optical signal are coupled to the fundamental-mode and higher-order mode waveguides at different points along the length of the grating such that chromatic dispersion is corrected at the transmitting end. The invention also provides a method for compensating for chromatic dispersion using this apparatus.